Self-propelled or powered wheelchairs have improved the mobility/transportability of the disabled and/or handicapped. Whereas in the past, disabled/handicapped individuals were nearly entirely reliant upon the assistance of others for transportation, the Americans with Disabilities Act (ADA) of June 1990 has effected sweeping changes to provide equal access and freedom of movement/mobility for disabled individuals. Notably, various structural changes have been mandated to the construction of homes, offices, entrances, sidewalks, and even parkway/river crossing, e.g., bridges, to include enlarged entrances, powered doorways, entrance ramps, curb ramps, etc., to ease mobility for disabled persons in and around society.
Along with these societal changes, the industry has created longer-running and stable power wheelchairs. Various technologies, initially developed for other industries, are being successfully applied to power wheelchairs to enhance the ease of control, improve stability, and/or reduce wheelchair weight and bulk. Innovations have also been made in the design of the wheelchair suspension system, e.g., active suspension systems, which vary spring stiffness to vary ride efficacy, have also been used to improve and stabilize power wheelchairs.
One particular system which has gained popularity/acceptance is mid-wheel drive power wheelchairs, and more particularly, such power wheelchairs with anti-tip systems. Mid-wheel drive power wheelchairs are designed to position the rotational axes of the drive wheels adjacent the center of gravity (of the combined occupant and wheelchair) to provide enhanced mobility and maneuverability. Anti-tip systems enhance stability of the wheelchair about its pitch axis and, in some of the more sophisticated designs, improve the obstacle or curb-climbing ability of the wheelchair. Such mid-wheel drive power wheelchairs having anti-tip systems are disclosed in Schaffner et al. U.S. Pat. Nos. 5,944,131 and 6,129,165, both assigned to Pride Mobility Products Corporation of Exeter, Pa.
While such designs have improved the stability of power wheelchairs, designers thereof are continually being challenged to examine and improve wheelchair design and construction. For example, the Schaffner '131 patent discloses a mid-wheel drive wheelchair having a passive anti-tip system. The passive anti-tip system functions principally to stabilize the wheelchair about its pitch axis, i.e., to prevent forward tipping of the wheelchair. The anti-tip wheel is pivotally mounted to a vertical frame support about a pivot point which lies above the rotational axis of the anti-tip wheel. As such, the system requires that the anti-tip wheel impact a curb or other obstacle at a point below its rotational axis to cause the wheel to “kick” upwardly and climb over the obstacle.
The Schaffner '165 patent discloses a mid-wheel drive power wheelchair having an anti-tip system which is “active” (that is, responsive to torque applied by the drive motor or pitch motion of the wheelchair frame) to vary the position of the anti-tip wheels, thereby improving the wheelchair's ability to climb curbs or overcome obstacles. More specifically, the active anti-tip system mechanically couples the suspension system of the anti-tip wheel to the drive assembly such that the anti-tip wheels displace upwardly or downwardly as a function of the magnitude of: the torque applied by the drive assembly, the angular acceleration of the frame and/or the pitch motion of the frame relative to the drive wheels.
FIG. 1 is a schematic of one variation of the anti-tip system disclosed in the Schaffner '165 patent. The drive assembly for the drive wheel 106 and the suspension for the anti-tip system 110, are mechanically coupled by a longitudinal suspension arm 124, pivotally mounted to the main structural frame 103 about a pivot 108. A drive assembly is mounted to the suspension arm 124 at one end and an anti-tip wheel 116 is mounted to the other. In operation, torque from a drive motor 107 results in relative rotational displacement of the drive assembly 107 about the pivot 108. The relative motion therebetween, in turn, effects rotation of the suspension arm 124 about the pivot 108 in a clockwise or counterclockwise direction, depending upon the direction of the applied torque. Upon an acceleration or increased torque input (as may be required to overcome or climb an obstacle), counterclockwise rotation of the drive assembly 107 will effect an upward vertical displacement of the respective anti-tip wheel 116. Consequently, the anti-tip wheels 116 are “actively” lifted or raised to facilitate such operational modes, e.g., curb climbing. Alternatively, deceleration causes a clockwise rotation of the drive assembly 107, thus effecting a downward vertical displacement of the respective anti-tip wheel 116. The downward motion of the anti-tip wheel 116 assists to stabilize the wheelchair when traversing downwardly sloping terrain or deceleration. Again, the anti-tip system “actively” responds to a change in applied torque to vary the position of the anti-tip wheel.
Another wheelchair suspension/anti-tip system, illustrated in U.S. Patent Application Publication No. 2004/0060748, assigned to Invacare Corporation, employs an arrangement of arms that displace an anti-tip wheel in two directions. A four-bar linkage arrangement is produced to raise the anti-tip wheel when approaching or climbing an obstacle while, at the same time, causing the anti-tip wheel to automatically move rearwardly to alter the angle of incidence of the wheel.